1. Field of the Invention
This invention relates to a gear shift control apparatus of a vehicle that executes gear shift control of an automatic transmission and, in particular, relates to a technique for executing a gear shift of the automatic transmission using a gear shift model.
2. Description of Related Art
Automatic transmissions having a plurality of engaging devices which transmit rotation and torque between an input shaft that receives power from a drive power source and an output shaft that transmits the power to a drive wheel and executing a gear shift by switching between engagement and release of the engaging devices are widely available. Generally, with such an automatic transmission, adaptation of a demanded value (in other words, a control operation amount) of an operated element (for example, torque) is performed on a control object for each gear stage while performing evaluation on an actual vehicle, and a gear shift is executed based on the adaptation result using a control operation amount determined from a control map obtained in advance for each gear stage. However, as the number of shift stages in automatic transmissions increase, significantly greater effort is required by such adaptation work and, as a result, it is becoming difficult to adopt modes of gear shift control that are based, on a control map. Therefore, gear shift model control that is a mode of gear shift control based on a motion equation of each rotating element constituting an automatic transmission is proposed. With such gear shift model control, a control operation amount is uniquely determined by solving a motion equation obtained in advance based on a variation mode (a gear shift target value) that is desirably achieved as a result of a gear shift and the determined control operation amount is used to execute a gear shift. For example, Japanese Patent Application Publication No. 2000-97325 (JP 2000-97325 A) describes, with respect to inertia phase control, a technique for executing a gear shift by setting a target value of an input shaft rotation speed of a transmission as a gear shift target value and calculating a demanded value of an engaging-side clutch torque as a control operation amount, and a technique for executing a gear shift by setting respective target values of an input shaft rotation speed and an output shaft torque of a transmission as gear shift target values and calculating a demanded value of an engaging-side clutch torque and a demanded value of a releasing-side clutch torque as control operation amounts using a gear shift model.
With the techniques described in JP 2000-97325 A, a gear shift is executed by either operating one control object with respect to one gear shift target value or operating two control objects with respect to two gear shift target values. However, with the techniques described in JP 2000-97325 A, since hydraulic pressure of a releasing-side engaging device is reduced towards release and subsequently temporarily increased once again toward engagement in order to cancel an inertia torque in an inertia phase (in other words, in order to prevent an output shaft torque in an inertia phase from substantially varying), there is a possibility that gear shift completion may be delayed and that drivability may decline. Meanwhile, there is a conventional method of so-called engine torque down control in which engine torque is temporarily reduced in an inertia phase in order to cancel the inertia torque. However, with the technique described in JP 2000-97325 A, the engine is not incorporated into the motion equation as a control object. In other words, with the technique described in JP 2000-97325 A, since the motion equation is solved with respect to a random engine torque, the gear shift model control described in JP 2000-97325 A is incapable of canceling an inertia torque by engine torque down control in place of temporarily increasing hydraulic pressure at the releasing-side engaging device. In this case, while engine torque down control can be executed separate from gear shift model control, such a procedure causes the entire gear shift model control to collapse and requires a solution to be once again derived from the motion equation, and may end up delaying gear shift completion or increasing gear shift shock which may cause a decline in drivability. On the other hand, attempting to uniquely determine an engine torque as a control operation amount by gear shift model control produces three control operation amounts with respect to two gear shift target values. As a result, the motion equation cannot be solved and a gear shift of the automatic transmission using gear shift model control can no longer be executed.
Problems such as those described above are not conventional, and no proposals have been made so far regarding appropriately setting a constraint for solving a motion equation when there are three control operation amounts with respect to two gear shift target values so as to enable a prescribed gear shift model to accommodate all gear shift patterns (gear shift modes) including power-on upshift, power-off upshift, power-on downshift, and power-off downshift. In contrast, this invention proposes a new technique for appropriately setting a constraint to solve a motion equation (refer to international application previously filed by the applicant but yet to be published at this time (International Application No.: PCT/JP2012/069408)). In addition, this invention proposes a technique which is based on, and further improves, the new technique described above.